Natsuko, do you live with your family or are you on your own? If you live with them, than as catxx said, everyone in that household needs to be a part of that dog's training. You can have a dog that knows every trick in the book, is perfectly well behaved when you're around, but take you out of the picture, and he may not listen to people who havent'worked with him. The first issue is consistancy. When the dog jumps, you do one thing which tells him that jumping is not okay, that he wont' get the attention or reaction. But then a member of your family comes along and does something else that may actually be encouraging the dog to jump. Your dog will get mixed signals making training harder overall and meaning that he won't listen to all of you the same way. Even if its just a matter of telling your family to do or not to do certain things when the dog jumps or mouths, that's a start. But you all have to use the same commands, the same hand signals, the same body language so as not to confuse your dog. And when you start obedience training again, you'll need to teach the family the same things you learned in class.
If you live on your own, tell your family when they come over and you're not there, to do whatever it is you're using to deter your dog from jumping. Our trainer said that she has a sign outside her door that says "Do not pet the dogs when you come in the house." Basically, she was training her dogs that when visitors come, its not that big a deal, but she was also training her visitors to not give her dogs the attention and reaction they were seeking.
Now, I take it the dog has run of the house when no one is home. Maybe you could try using a dog crate. Then he won't be right there at the door when someone comes in. You can give him a bit to calm down before you let him out. Just another option to look in to.